1. Field of the Invention
The present invention relates generally to burner control, and more specifically to methods and apparatus for controlling combustion efficiency in burners.
2. Description of the Related Art
Numerous industrial processes such as glass or fritt melting, ferrous and nonferrous materials smelting, ladle preheating, billets reheating, waste incineration and vitrification, crude oil refining, petrochemical production, power plants, and the like use burners as the primary source of energy, or as an auxiliary source of energy. These burners possess one or more inlets for fossil fuels of high calorific value such as natural gas, liquefied petroleum gas, liquid hydrocarboneous fuel, and the like, which are combusted to produce heat. Some burners also comprise inlets for low calorific content gases or liquids that need to be incinerated. The fuels are burned in a combustion chamber where the energy that is released by the combustion is transferred to the furnace load. The combustion requires an oxidant, such as air, oxygen enriched air, or oxygen, and the oxidant is preferably preheated. The oxidant is also supplied by the burners.
Precise and reliable control of the combustion is very important for the efficiency and the safety of industrial processes, as will be understood by those skilled in the art.
For instance, it is well known that combusting a fuel with excess oxidant yields higher nitrogen oxides (NOx) emission rates, especially when the oxidant is preheated or when the oxidant is pure oxygen. On the other hand, incomplete combustion of a fuel generates carbon monoxide (CO). Both NOx and CO are very dangerous pollutants, and the emission of both gases is regulated by environmental authorities.
Combustion of a fuel with an uncontrolled excess amount of air can also lead to excessive fuel consumption and increase the production cost of the final product.
Safety of operation is an essential characteristic expected from all industrial combustion systems. Automated control of the presence of the flame in the combustion can be used to stop the flow of oxidant when the fuel flow is suddenly interrupted.
Commercially available UV flame detectors can be used to control the status (flame on or off) of a flame. However, this type of combustion control device does not give any information on the combustion mixture. It is impossible to know whether the burner is operated under fuel rich (excess of fuel, equivalence ratio greater than 1), fuel lean (excess of oxidant, equivalence ratio less than 1), or stoichiometric (exact amounts of fuel and oxidant to obtain complete combustion of the fuel, equivalence ratio equal to 1). UV flame detectors are typically self contained devices that are not always integrated in the burner design.
Endoscopes are also often used in the industry to visually inspect flames, and their interaction between the furnace load. They are generally complicated and expensive pieces of equipment that require careful maintenance. To be introduced into very high temperature furnaces, they require external cooling and flushing means: high pressure compressed air and water are the most common cooling fluids. When compressed air is used, uncontrolled amounts of air are introduced in the furnace and may contribute to the formation of NOx. Water jackets are subject to corrosion when the furnace atmosphere contains condensable vapors.
Control of the combustion ratio at a burner can be performed by metering the flows of fuel and oxidant, and using valves (electrically or pneumatically driven) controlled by a programmable logic controller (PLC). The ratio of oxidant to fuel flow is predetermined using the chemical composition of the natural gas and of the oxidant. To be effective, the flow measurement must be very accurate and calibrated on a regular basis, which is not always the case, especially when the oxidant is air. This situation often leads the furnace operator to use a large excess of air to avoid the formation of CO. This feed-forward combustion control strategy does not account for the air intakes that naturally occur in industrial furnaces and bring unaccounted quantities of oxidant into the firebox, nor does this control scheme account for the variation of the air intakes caused by furnace pressure changes. Another drawback is that the response time of the feed-forward regulation loop is generally slow, and can not account for cyclic variations of oxidant supply pressure and composition that occur when the oxidant is impure oxygen, for example as produced by a vacuum swing adsorption unit or membrane separator. Yet another drawback of the feed-forward control of combustion ratio is that the PLC should be reprogrammed at every occurrence of a change in natural gas supply and composition.
Placing an in-situ oxygen sensor at the furnace exhaust can provide a feed-back control solution for global combustion ratio control. However, zirconia sensors for oxygen that are commercially available have limited lifetime and need to be replaced frequently. One difficulty met when using these sensors is a tendency to plug, especially when the exhaust gases contain volatile species, such as in a glass production furnace. When the furnace possesses more than one burner, a drawback of global combustion control is that it is not possible to know whether each individual burner is properly adjusted or not. This technique also has long response times due to the residence times of the furnace gases in the combustion chamber, which can exceed 30 seconds.
Continuous CO monitoring of the flue gas, for example in so-called post combustion control of an electric arc furnace, provides another means of controlling the combustion. It involves the use of a sophisticated exhaust gas sampling system, with separation of the particulate matter and of the water vapor. Although very efficient, these techniques are not always economically justified.
Other combustion control devices use acoustic control of flames. Most of these systems were developed for small combustion chambers in order to avoid extinction of flames, and are triggered by instabilities of flames.
The light emission observed from flame is one of the most characteristic features providing information on the chemical and physical processes taking place. Monitoring the flame light emission can be easily performed in well controlled environments typically found in laboratories. However, implementing flame light emission monitoring on industrial burners used on large furnaces is quite difficult in practice, resulting in a number of problems. First, optical access is necessary which requires positioning of a viewport in a strategic location with respect to the flame for collecting the flame light emission. Second, the plant environment is difficult because of excessive heat being produced by the furnace. Typical optical ports on a furnace can have temperatures in excess of 1000 .degree. C., thus necessitating the need for water cooled or high flow-rate gas cooled probes for use either in or near the furnace. Finally, these environments tend to be very dusty which is not favorable for the use of optical equipment except with special precautions, such as gas purging over the optical components.
While currently available systems have been able to achieve some degree of control over the combustion in a burner, there is a need for a fast response time control apparatus that avoids the previously described problems.